Audio follow video mixers are utilized in professional recording applications to process several channels of audio input to a mixer console. Such mixer systems may be operated manually or under control of signals supplied by a video editor coupled to the mixer console. In a typical operation, two audio channels are mixed by performing a cross fade between two active channels. This operation involves fading out a first audio signal, while fading in a second. More particularly, a FADE OUT operation involves attenuating an audio signal from a first amplitude to a second amplitude; conversely, a FADE IN operation involves amplifying an audio signal from a first level to a second level. This may be achieved by supplying the audio signal to be attenuated to a voltage control amplifier (VCA) device which attenuates the audio signal according to an analog control voltage supplied thereto.
Several commercially available mixer devices are equipped with control members which are adjusted to set both the maximum and minimum levels of a particular audio channel in the mixer during a FADE operation. Typically, a first potentiometer is used to control the maximum level of the audio channel and a second potentiometer is used to control the minimum level of the audio channel obtained during a FADE.
A particularly advantageous configuration for performing FADE IN and FADE OUT operations is described in commonly-owned patent application Ser. No. 07/683,887, entitled "Fader Depth Control Apparatus," the disclosure of which is hereby incorporated by reference. As shown in FIG. 1, this configuration utilizes a main fader member (for example, a linear potentiometer) to control the maximum level at which an audio signal is amplified, and a secondary fader member to determine the absolute level by which an audio signal is attenuated (that is, the minimum level of the audio signal after a FADE OUT operation).
This configuration is shown in detail in FIG. 2, wherein a VCA control voltage is applied through a switch SW which can be in a first or second position. As shown, the switch SW is in a position corresponding to a channel activation position such that VCA control voltages may be supplied through the switch to IC 5. IC 5 is a buffer for the VCA control signal. In another position, the channel is deactivated such that no control voltages may be supplied to this channel. In the circuit shown, the VCA control signal is a direct voltage signal which varies from a first level corresponding to maximum attenuation (typically +5 V) to a second level corresponding to maximum gain (typically 0 V). The VCA control signal may be supplied from a video editor to the mixer console via an editor interface (not shown).
The output of IC 5 is provided to a depth of fade control 10. This depth of fade control supplies the VCA control signal to the non-inverting input of IC 6. The depth of fade control 10 is illustrated as a variable resistance voltage divider (such as a linear potentiometer) which determines the amount of attenuation of the VCA control signal. For example, in the position indicated as "-.infin." the VCA control signal from the editor (not shown) is sent to the IC 6 without change. In other positions, the depth of fade control 10 lowers the level of the VCA control signals. For example, at the ground position, the resulting control signal of 0 V corresponds to full gain of the audio signal input to the VCA. At an intermediate position, located between -.infin. and full gain, the depth of fade control provides attenuation of the audio signal by some factor between full attenuation and maximum gain.
A voltage divider 30 functions in a known manner as a reference voltage generator, while a main fader is shown as element 20. Collectively, ICs 1, 2, 3 and 4 function as a curve shaping circuit in a known manner to shape the curve for the channel fader 20. The signal from the fader 20, after being subject to the curve shaping circuit, is supplied to IC 7. Both IC 6 and IC 7 are precision rectifiers which function similarly to an analog OR gate. These elements operate to compare the main fader signal and the editor signal. The larger of the main fader signal and the fader depth signal (by voltage) is passed on to a buffer and voltage adjuster (IC 8). The output of IC 8 is coupled to the control input of a VCA which controls the gain of an input audio channel according to the larger of either the main fader signal or the editor signal. In this way, the depth of fade level (determining the minimum level of amplification during a FADE) and the main channel fader level (determining the maximum level of amplification during a FADE) are set separately and independently.
This configuration has been incorporated into the MXP-390 series mixer produced by the assignee of this invention. While successfully providing an independent means to adjust the absolute minimum of a signal to be faded out in a FADE operation, MXP-390 series mixers suffer a drawback in that the position of the depth of fade control member (the secondary fader shown in FIG. 1) may not be readily apparent. Although symbols are printed on the MXP-390 series mixer console to indicate the position of the depth of fade member, such symbols are difficult to read, and thus fail to clearly indicate when the depth of fade member is slightly out of a full attenuation position.
For example, in a typical operation the mixer console is configured to cross fade from a first channel A to a second channel B. Thus, it is intended that an audio channel A should be gradually attenuated from a first level, usually unity gain, to maximum attenuation (-.infin. dB), while a second audio channel B is amplified from maximum attenuation to full gain. That is, channel A should FADE OUT and channel B should FADE IN. However, if it is unnoticed by the operator that the depth of fade control member corresponding to input audio channel A is not in a position permitting full attenuation, audio channel A will not be fully attenuated at the end of the cross fade operation. Instead, an audio signal corresponding to channel A will remain in the output path leading from the VCA. Thus, if the mixed (cross faded) output signal is recorded, for example, an unwanted audio signal which includes channel A may be present in the recorded signal. Moreover, since such an unwanted audio signal is usually substantially attenuated, it is difficult to detect during typical recording monitoring, but may be detected after distribution of the recording. Therefore, this problem may lead to unnecessary costs in re-recording.
Moreover, this problem is not merely confined to MXP-390 series mixers. Rather, other mixers which provide a separate depth of fade control member to adjust the maximum attenuation of an audio signal suffer would benefit from a clearer indication of the position of the depth of fade control member.
Accordingly, there is a need to provide an improved depth of fade control device which provides a clear indication to an operator of the position of the depth of fade control member.